In the past, the practical altitude ceiling at which an airplane can operate has been limited by the maneuver margin to stall (typically 1.3 g to initial buffet), which historically has been primarily a function of the efficiency of the aircraft's wing. Optimization of aerodynamic design techniques in manufacturing processes introduced in the 1990s have resulted in substantial increases in the efficiency of airplane wings. In some aircraft, the efficiency of the wing is such that the practical altitude ceiling during certain turn maneuvers is limited not by the ability of the wings to continue providing sufficient lift, but instead by the thrust available from the engines. Simplistically, the airplane engines run out of energy before the wings run out of lift. This situation is further aggravated by the marked decrease in available thrust that occurs in modern, fuel efficient high-bypass ratio turbofan engines at high altitudes and Mach numbers.
Some designers have addressed this problem by restricting the maximum altitude at which the airplane can operate. This unnecessarily penalizes the performance of the airplane and can result in increased fuel consumption, reduced payload, and increased en route flight time to the planned destination. Other designers have restricted the minimum speed at which the airplane operates. This also unnecessarily penalizes the performance of the airplane, resulting in increased fuel consumption. Enforcing the maximum altitude and/or minimum speed at which the airplane operates not only carries economic penalties, but such restrictions still may not prevent the airplane from operating in a regime where a turn maneuver may result in a potential loss of altitude or airspeed.
Other designers have incorporated a manual bank angle limit selector switch into their particular autopilot flight director systems. This switch provides for manual pilot intervention when operating an autopilot flight director system in a heading or track mode. Pilot selection of a bank angle limit of between 5.degree. and 30.degree. is typically provided. The manual bank angle limit selector does not affect the bank angle limit, though, when operating the airplane in a fully automatic flight mode, where airplane roll guidance commands are generated exclusively by a flight management system (FMS).
Thus, a need exists for a method and means to effectively maneuver an airplane in a potentially thrust-limited turn condition so that flight can be sustained at the desired altitude and the desired air speed when operating in autoflight. The ideal method should be preventative in nature to avoid any uncommanded loss of energy (altitude and/or airspeed). The present invention is directed to fulfilling this need.